Mountain weather phenomena: fogbows, inversions and Brocken spectres

21st December 2016

Our guide to the different kinds of weather phenomena you can discover in the hills and the conditions that bring the best chances of finding them

Temperature inversions (main image)

Your best chance of witnessing a temperature inversion is on a morning with high pressure and low fog after a cold, still night. If you’re lucky, as you climb upward, you’ll break through the top of the band of cloud into clear skies, with the cloud cloaking the landscape below you.

Brocken spectres

Brocken spectres look more eerie than straightforward shadows as they appear to be far away yet larger than you expect, due to the effect of cloud on your depth perception. Usually a glory – a circle of rainbow – appears around the head of the shadow. Spectres can also have an almost three-dimensional appearance thanks to the movement of layers of cloud. You need to be standing above the upper surface of a cloud (for example, during a cloud inversion) and for the sun to be behind you, matching the angle of the slope, creating a long shadow. Gently rounded summits can offer favourable angles, but it’s never possible to guarantee a sighting of a Brocken Spectre, making them even more special when they do occur.

Fogbows

By Eric Whitehead

Fogbows (or cloudbows or white rainbows) are caused by similar circumstances to rainbows but because the water droplets are so small, they diffract the light and cause it to overlap into white rather than refract into colours. The combination of thin fog and bright sunshine is necessary to see a fogbow and the crucial time is the moment when the sun breaks through the fog. The bow is centered around the anti-solar point (directly opposite the sun).

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